Method for treating a cast iron workpiece and workpiece formed thereby

ABSTRACT

A method for treating a cast iron workpiece to increase a useful life thereof includes machining the workpiece to provide a finish surface thereon and deforming the finish surface of the workpiece by rubbing the finish surface against a blunt tool ( 80,80′ ), thereby forming a nanocrystallized surface layer ( 70 ). The workpiece is nitrocarburized, the nanocrystallized surface layer accelerating diffusion of nitrogen atoms and carbon atoms therethrough. The nitrocarburizing taking place: i) if the workpiece is stress relived prior to machining, for about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., or ii) if the workpiece is not stress relieved prior to machining, for about 5 hours to about 10 hours at a temperature ranging from about  370°  C. to about  450°  C. The nitrocarburizing renders the nanocrystallized surface layer into i) a friction surface, or ii) a corrosion-resistant surface.

TECHNICAL FIELD

The present disclosure relates generally to methods for treating cast iron workpieces and workpieces formed thereby.

BACKGROUND

Cast iron materials may be used in applications where resistance to surface wear from friction is desirable. Untreated cast iron materials generally tend to corrode when exposed to the environments in which they are used. Some surface treatments, e.g., painting, tend to wear off quickly and/or may be deleterious to proper functioning of the cast iron materials.

SUMMARY

A method for treating a cast iron workpiece to increase a useful life thereof includes machining the workpiece to provide a finish surface thereon and deforming the finish surface of the workpiece by rubbing the finish surface against a blunt tool, thereby forming a nanocrystallized surface layer. The workpiece is nitrocarburized, the nanocrystallized surface layer accelerating diffusion of nitrogen atoms and carbon atoms therethrough. The nitrocarburizing taking place: i) if the workpiece is stress relieved prior to machining, for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., or ii) if the workpiece is not stress relieved prior to machining, for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C. The nitrocarburizing renders the nanocrystallized surface layer into i) a friction surface, or ii) a corrosion-resistant surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of examples of the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear.

FIG. 1 is a perspective view of a disc brake assembly in an example of the present disclosure;

FIG. 2 is a side view of a drum brake assembly in an example of the present disclosure;

FIG. 3 is a schematic perspective view showing an example of a workpiece and a tool operating thereon;

FIG. 3A is an enlarged cross-sectional schematic view of a portion of the workpiece and tool of FIG. 3, showing the tool forming the nanocrystallized surface layer;

FIG. 4 is an enlarged cross-sectional schematic view showing an example of the workpiece in a nitrocarburizing environment;

FIG. 4A is a schematic depiction of a section view showing an example of a compound layer after ferritic nitrocarburization at a microscopic enlargement;

FIG. 5 is a perspective view of a brake disc in an example of the present disclosure;

FIG. 6 is a scanning electron microscope (SEM) image, similar to the view of FIG. 4, but showing an example of an actual workpiece depicting the microstructure of the substrate and the nanocrystallized surface layer;

FIG. 7 is a perspective view of a brake drum in an example of the present disclosure;

FIG. 8 is a perspective view showing the inside of the brake drum depicted in FIG. 7;

FIG. 9 is a perspective view of a drum-in-hat rotational member;

FIG. 10 is a cross sectional view of the drum-in-hat rotational member depicted in FIG. 9;

FIG. 11A is a flow diagram depicting an example of a method according to the present disclosure;

FIG. 11B is a flow diagram depicting another example of a method according to the present disclosure;

FIG. 12 is a perspective view of a shaft in an example of the present disclosure;

FIG. 13 is a perspective view of an engine block cylinder liner in an example of the present disclosure; and

FIG. 14 is a perspective view showing the inside of the brake drum depicted in FIG. 8 with a schematic view of a tool operating thereon.

DETAILED DESCRIPTION

Examples of the present disclosure advantageously provide a surface nanocrystallization process for faster or more energy efficient ferritic nitrocarburizing (FNC) treatments of cast iron.

Generally, methods according to examples of the present disclosure include surface nanocrystallization by, e.g., deforming against a blunt tool, and accelerated diffusion of nitrogen and carbon atoms through the nanocrystallized surface layer, to form a substantially rust-free and high wear/fatigue resistant case on cast iron components/workpieces.

It is to be understood that in examples of the present disclosure, the deformation against the blunt tool is severe, plastic deformation local to the location of contact between the blunt tool and the workpiece. The deformation occurs substantially without forming chips and without removing material in the process of deformation. Further, the local deformation of examples of the present disclosure is distinct from global deformation that would occur in wire drawing or sheet-metal rolling. Although the deformation of the present disclosure occurs in the vicinity of the blunt tool, a large surface of a workpiece may be nanocrystallized by systematically applying the blunt tool to the entire surface. In an example, a cylindrical surface may be nanocrystallized by rotating the cylinder while moving the blunt tool along the cylindrical axis. In the example, the blunt tool would take a spiral path over the entire surface of the cylinder. It is to be further understood that more than one pass may be made over the finish surface with the blunt tool. In an example, four passes are made over the finish surface with the blunt tool.

Conventional Ferritic NitroCarburizing (FNC) usually takes about 5 to 6 hours at about 570° C. to obtain a 10 micron thick hard layer on the surface of metallic parts (for example, brake rotors) for better wear, fatigue and corrosion resistance. In contrast, examples of the method of the present disclosure may advantageously reduce FNC time down to about 1 to 2 hours to achieve the same hard layer thickness and therefore considerably reduce the processing energy cost.

Referring first to FIG. 11A, an example 100 of the method of the present disclosure includes casting a cast iron (e.g., grey cast iron, nodular cast iron, etc.) workpiece, as shown at reference numeral 102; stress relieving the cast iron workpiece, as shown at reference numeral 104; machining the workpiece to provide a finish surface thereon, as shown at reference numeral 106; deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against a blunt tool (described further herein), thereby forming a nanocrystallized surface layer at the finish surface, as shown at reference numeral 108; and nitrocarburizing the workpiece for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., as shown at reference numeral 110.

The nanocrystallized surface layer accelerates/facilitates diffusion of nitrogen atoms and carbon atoms therethrough. It is to be understood that the nanocrystallized surface layer (described further below at reference numeral 70) has any suitable thickness. However, in an example of the present disclosure, the thickness of nanocrystallized surface layer 70 ranges from about 3 μm to about 15 μm. In a further example, the thickness of nanocrystallized surface layer 70 is about 8 μm.

Referring now to FIG. 11B, another example 100′ of the method of the present disclosure includes casting a cast iron workpiece, as shown at reference numeral 102; machining the workpiece to provide a finish surface thereon, as shown at reference numeral 106; deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against a blunt tool (described further herein), thereby forming a nanocrystallized surface layer at the finish surface, as shown at reference numeral 108; and nitrocarburizing the workpiece for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C., as shown at reference numeral 110′. The nanocrystallized surface layer accelerates/facilitates diffusion of nitrogen atoms and carbon atoms therethrough.

In each of the examples above of the present method, the FNC renders the nanocrystallized surface layer into i) a friction surface (described further below at reference numerals 46, 46′), or ii) a corrosion-resistant surface (e.g., reference numerals 86, 86′ in FIGS. 4A and 12). As used herein, it is to be understood that a “friction” surface may also be a corrosion-resistant surface (in addition to being wear-and fatigue-resistant); however, a “corrosion-resistant” surface is not necessarily a friction surface. It is to be further understood that, in an example, the “corrosion-resistant” surface may be a free (non-contact) surface.

This formation of the nanocrystallized surface layer prior to FNC allows a higher diffusion rate of nitrogen and carbon into the cast iron workpiece, which leads to a considerably more efficient FNC process.

Without being bound to any theory, it is believed that at least the following three aspects are improved with methods of the present disclosure: 1. at conventional FNC temperatures (e.g., method 100), the FNC processing time may be reduced down to about 1 hour to 2 hours (from the conventional 5 to 6 hours); 2. alternatively (e.g., method 100′), FNC may be performed at a low temperature at which conventional FNC cannot thermodynamically create a hard nitride layer. This low temperature treatment may lead to a better dimensional stability, thereby eliminating the need for a stress relief step in some instances; and 3. the surface nanocrystallized microstructure may itself contribute to better wear and fatigue performance of the workpiece.

Referring now to FIG. 3, an example of a workpiece (e.g., a rotational member/brake rotor 12, 39) and a hard, blunt tool 80 operating thereon is shown (this takes place after the finish machining of the cast iron workpiece). The workpiece is depicted rotating about an axis while the tool 80 including a pellet 82 (made, e.g., from an iron-tungsten alloy, silicon carbide, boron nitride, titanium nitride, diamond, hardened tool steel, or the like) in contact with the finish surface. It is to be understood that, according to examples of the present disclosure, the workpiece, the tool 80, 80′ (see FIG. 14), or both may be rotating while the tool 80, 80′ is operating on the workpiece. Still further, in examples of the present disclosure, neither the tool 80, 80′ nor the workpiece may be rotating, but rather the tool 80, 80′ may be moving, e.g., transversely forward and backward while the workpiece is translated longitudinally, or vice versa. It is to be yet further understood that other methods of bringing the tool 80, 80′ into deforming rubbing contact with the workpiece are contemplated as being with the purview of the present disclosure.

The tool 80, 80′ applies a deforming force to the finish surface of the workpiece. In an example, the blunt tool 80, 80′ may be advanced by rotating a leadscrew that controls the advancement of the blunt tool 80, 80′ into a rotating finish surface of the workpiece by about 0.03 mm beyond first contact between the rotating workpiece and the blunt tool 80. It is to be understood that advancing the blunt tool 80, 80′ by about 0.03 mm does not necessarily create penetration of 0.03 mm in part because of elastic deformation of the workpiece, the pellet 82, and the holding fixture of the blunt tool 80. Further, the pellet 82 is not sharp and does not cut the finish surface. Blunt tool 80 reorganizes the crystal structure of the finish surface substantially without removing material therefrom. It is to be understood that the deformation of the finish surface may not be visible to the naked eye. However, a change in the reflective properties of the finish surface may be observable to the naked eye.

In an example, the tool 80 may cause pellet 82 to vibrate relative to the workpiece (as indicated by the double sided arrow V shown in phantom in FIG. 3). The vibration may be accomplished at ultrasonic frequencies (e.g., about 10,000 Hz to about 100,000 Hz).

FIG. 3A is an enlarged schematic view showing the pellet 82 of the tool 80 forming a nanocrystallized surface layer 70 at the surface of the workpiece substrate 84. It is to be understood that the pellet 82 may be a sphere, a spherical cap, a roller, a parabolic shape, or any shape that makes a local indentation on the finish surface and creates heavy deformation when operating on the workpiece (e.g., by rotating the workpiece thereagainst).

In examples of the present disclosure, a coolant may be applied to the tool and/or the workpiece. It is to be understood that the heat transfer properties of the coolant may improve tool life and nanocrystallization characteristics, however, lubrication may have deleterious effects on the method disclosed herein in some instances. Examples of suitable coolants are water, air, carbon dioxide gas, and nitrogen gas which generally do not have high lubricity but have good heat transfer characteristics.

FIG. 4 is an enlarged cross-sectional schematic view showing an example of the workpiece in a nitrocarburizing environment. The nanocrystallized surface layer 70, due, e.g., to a large number of grain boundaries, facilitates diffusion of the nitrogen and carbon therethrough, toward the base material substrate 84 during the FNC process(es) 110, 110′.

Examples of the methods of the present disclosure are relatively simple to execute and can be applied to many workpieces (one example of which is a component with axial symmetry that can be rotated during metal work, e.g., components having a disc shape or round bar shape). FIG. 12 depicts a cast iron shaft produced according to an example of the present disclosure. The cast-iron shaft 37 has a corrosion-resistant surface 86′ formed according to an example of the present disclosure. FIG. 13 depicts a cast iron engine block cylinder liner produced according to an example of the present disclosure. The cast iron engine block cylinder liner 35 has an internal surface 87 (formed according to an example of the present disclosure) that resists wear from friction by piston rings and resists corrosion.

An example of a cast iron workpiece is a rotational member of a vehicle brake. A brake 10 is an energy conversion system used to retard, stop, or hold a vehicle. While a vehicle in general may include spacecraft, aircraft, and ground vehicles, in this disclosure, a brake 10 is used to retard, stop, or hold a wheeled vehicle with respect to the ground. More specifically, as disclosed herein, a brake 10 is configured to retard, stop, or hold at least one wheel of a wheeled vehicle. The ground may be improved by paving.

A vehicle brake 10 may be a disc brake 20, drum brake 50, and combinations thereof. FIG. 1 depicts an example of a vehicle brake, in particular, a disc brake 20. In a disc brake 20, a rotational member 12 is typically removably attached to a wheel (not shown) at a wheel hub 40 by a plurality of wheel studs 24 cooperatively engaged with lug nuts (not shown). The rotational member 12 in a disc brake 20 may be known as a brake disc (or rotor) 39. The rotor 39 may include vent slots 38 to improve cooling and increase the stiffness of the brake disc 39. When hydraulic fluid is pressurized in a brake hose 34, a piston (not shown) inside a piston housing 32 of a caliper 28, causes the caliper 28 to squeeze the brake disc 39 between brake pads 36, thereby engaging the disc brake 20. The brake pads 36 may include a friction material that contacts a friction surface 46 of the brake disc 39 when the disc brake 20 is engaged. If the wheel is rotating at the time the disc brake 20 is engaged, kinetic energy of the moving vehicle is converted to heat by friction between the brake pads 36 and the brake disc 39. Some of the heat energy may temporarily raise the temperature of the brake disc 39, but over time, the heat is dissipated to the atmosphere surrounding the vehicle.

Referring now to FIG. 2, an example of a drum brake 50 is shown. The rotational member 12′ is a brake drum 56 (see also FIGS. 7 and 8). The brake drum 56 is removably fastened to a wheel (not shown). The brake drum 56 may include fins 68 to improve cooling and increase the stiffness of the brake drum 56. When hydraulic fluid is pressurized in a wheel cylinder 52, a piston 54 causes the brake shoes 62 to press a brake lining 66 against the brake drum 56, thereby engaging the drum brake 50. It is to be understood that the brake lining 66 is a friction material. Alternatively, a drum brake 50 may be engaged mechanically by actuating an emergency brake lever 64 via an emergency brake cable 58. The emergency brake lever 64 causes the shoes 62 to press the brake lining 66 against the brake drum 56. If the wheel is rotating at the time the drum brake 50 is engaged, kinetic energy of the moving vehicle is converted to heat by friction between the brake lining 66 and the brake drum 56. Some of the heat energy may temporarily raise the temperature of the brake drum 56, but over time, the heat is dissipated to the atmosphere surrounding the vehicle.

FIG. 7 shows a perspective view of a brake drum 56 in an example of a rotational member 12′ . FIG. 8 is a rotated perspective view of the brake drum 56 shown in FIG. 7, showing an inside view of the brake drum 56. The friction surface 46′ is visible in FIG. 8. In examples of the present disclosure, the workpiece may be nanocrystallized in a process similar to the process disclosed herein with respect to FIG. 3 and FIG. 3A. As shown in FIG. 14, an example of the blunt tool 80′ may have a right angle configuration to provide access to the friction surface 46′ on an internal wall of the brake drum 56. When the friction surface 46′ is cylindrical as in the brake drum example depicted in FIGS. 7 and 8, the blunt tool 80′ (see FIG. 14) is advanced into the finish surface by moving the blunt tool 80′ radially outward and engaging the pellet 82 with the finish surface and transforming the finish surface into the nanocrystallized surface layer 70 (not shown in FIG. 14) and then into friction surface 46′ after ferritic nitrocarburization (FNC). As shown in both FIGS. 7 and 8, examples of a brake drum 56 may include fins 68.

It is to be understood that a disc brake 20 may be combined with a drum brake 50. As shown in FIGS. 9 and 10, a drum-in-hat rotational member 12′′ may be included in such a combination. In a drum-in-hat type brake, small brakeshoes may be mechanically/cable actuated as an emergency brake, while the flange portion acts as a typical disc brake.

The rotational member 12, 12′, 12′′ includes a friction surface 46, 46′ that is engaged by a friction material of the brake pad 36 or the brake shoe 62. As a brake is engaged to retard a vehicle, mechanical wear and heat may cause small amounts of both the friction material and the rotational member 12, 12′, 12″ to wear away. It may be possible to reduce the rate of wear of the rotational member 12, 12′, 12″ or the friction material by reducing the coefficient of friction between the two, but a lower coefficient of friction may make the brake 10 less effective at retarding the vehicle.

In cast iron, corrosion is mainly the formation of iron oxides. Iron oxides are porous, fragile and easy to scale off. Further, corrosion on a friction surface may be non-uniform, thereby deleteriously affecting the brake performance and useful life. Thus, corrosion may lead to undesirably rapid wear of the friction surface 46, 46′ and the corresponding friction material.

Ferritic nitrocarburization produces a friction surface 46, 46′ that resists corrosion and wear. In examples of the present disclosure, ferritic nitrocarburization is used to render the nanocrystallized surface layer 70 into a compound layer 70′ on the rotational member 12, 12′, 12″ of the workpiece (e.g., brake 10). In an example, rotational member 12, 12′, 12″ has a compound layer 70′ disposed at the friction surface 46, 46′, corrosion-resistant surface 86, 86′. The compound layer 70′ may have an exposed surface in contact with an atmosphere, for example, air.

As depicted in FIG. 4A, compound layer 70′ further may include an oxide layer 72 having Fe₃O₄ disposed at the exposed surface (friction surface 46, corrosion-resistant surface 86). An iron nitride layer 74 including epsilon Fe₃N iron nitride and gamma prime Fe₄N iron nitride may be generally subjacent the oxide layer 72 and containing a majority of epsilon Fe₃N iron nitride. Further, the oxide layer 72 may have a thickness 73 ranging from about 5% to about 50% of a thickness 75 of the iron nitride layer 74. As shown in FIG. 4A, a diffusion layer 77 is subjacent the iron nitride layer 74 and is a transition between the iron nitride layer 74 and a portion of the workpiece (e.g., rotational member) that is beyond the reach of ferritic nitrocarburization (not shown).

In an example, a ferritically nitrocarburized rotational member 12, 12′, 12″ having a friction surface 46, 46′ formed by methods of the present disclosure exhibits a friction material wear of less than 0.4 mm per 1000 stops at about 350° C. An experiment using the test procedure in Surface Vehicle Recommended Practice J2707, Issued Feb. 2005 by SAE International may be conducted. An Akebono NS265 Non Asbestos Organic (NAO) friction material may be used in the experiment.

Referring now to FIG. 5, a perspective view of a brake disc 39 in an example is shown. Rotational member 12 is a brake disc 39 with vent slots 38.

FIG. 6 is a scanning electron microscope (SEM) image showing an example of an actual workpiece depicting the microstructure of the workpiece substrate and the nanocrystallized surface layer (the thickness of the nanocrystallized surface layer 70 in this example is about 8 microns). A scale indicator is provided in FIG. 6 to facilitate estimation of relative sizes.

The workpiece/rotational member 12, 12′, 12″ may be made from cast iron. The friction surface 46, 46′ may exhibit a hardness of between about 56 HRC and about 64 HRC. Hardness is directly related to wear resistance.

Machining 106 may be accomplished by, for example, turning, milling, sand blasting, grit blasting, grinding, and combinations thereof.

It is to be understood that nitrocarburizing includes a gas nitrocarburizing process, a plasma nitrocarburizing process, or a salt bath nitrocarburizing process. The salt bath nitrocarburizing process may include immersing at least the friction surface 46, 46′ of the rotational member 12, 12′, 12″ into a nitrocarburizing salt bath, and then immersing at least the friction surface 46, 46′ of the rotational member 12, 12′, 12″ into an oxidizing salt bath.

It is to be understood that the rotational member 12, 12′, 12″ may include a brake disc 39, a brake drum 56, or a combination thereof.

Further, examples of the present methods 100, 100′ may improve corrosion resistance similarly to FNC methods performed without first forming nanocrystallized surface layer 70.

In summary, examples of the method of the present disclosure may reduce FNC cycle time by a factor of about 5 to 10 (e.g., reduced from about 5 to 6 hours to about 1 to 2 hours at 570° C.). Alternately, examples may enable low temperature FNC (reduced from 570° C. to about 400° C.-450° C.) to reduce part distortion. Examples of the present disclosure further produce workpieces with improved wear/fatigue resistance and corrosion resistance. Increased productivity is achievable compared to other surface nanocrystallization processes. For example, nanocrystallization by shot peening may require about 36 seconds per square centimeter. In sharp contrast, examples of the method disclosed herein may take about 2 seconds per square centimeter.

Numerical data have been presented herein in a range format. It is to be understood that this range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a time period ranging from about 5 hours to about 10 hours should be interpreted to include not only the explicitly recited limits of about 5 hours to about 10 hours, but also to include individual amounts such as 5.5 hours, 7 hours, 8.25 hours, etc., and sub-ranges such as 8 hours to 9 hours, etc. Furthermore, when “about” is utilized to describe a value, this is meant to encompass minor variations (up to +/−10%) from the stated value.

In describing and claiming the examples disclosed herein, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.

While several examples have been described in detail, it will be apparent to those skilled in the art that the disclosed examples may be modified. Therefore, the foregoing description is to be considered non-limiting. 

1. A method for treating a cast iron workpiece to increase a useful life thereof, the method comprising: either i) stress relieving the workpiece, or ii) refraining from stress relieving the workpiece; machining the workpiece to provide a finish surface thereon; deforming the finish surface of the workpiece by rubbing the finish surface against a blunt tool, thereby forming a nanocrystallized surface layer at the finish surface; and nitrocarburizing the workpiece, the nanocrystallized surface layer accelerating diffusion of nitrogen atoms and carbon atoms therethrough, the nitrocarburizing taking place: i) if the workpiece is stress relieved, for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., or ii) if the workpiece is not stress relieved, for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C., thereby rendering the nanocrystallized surface layer into i) a friction surface, or ii) a corrosion-resistant surface by the nitrocarburizing.
 2. The method as defined in claim 1 wherein the workpiece is a rotational member of a vehicle brake.
 3. The method as defined in claim 1 wherein the workpiece is a shaft or an engine block cylinder liner.
 4. The method as defined in claim 1 wherein machining is accomplished by a process selected from turning, milling, sand blasting, grit blasting, grinding, and combinations thereof.
 5. The method as defined in claim 1 wherein nitrocarburizing includes a gas nitrocarburizing process, a plasma nitrocarburizing process, or a salt bath nitrocarburizing process.
 6. The method as defined in claim 1 wherein the nitrocarburizing comprises: immersing at least the nanocrystallized friction surface of the workpiece into a nitrocarburizing salt bath; and then immersing the at least the nanocrystallized friction surface into an oxidizing salt bath.
 7. The method as defined in claim 1 wherein rubbing the finish surface against the blunt tool is accomplished by rotating the finish surface against the blunt tool.
 8. The method as defined in claim 7 wherein four passes are made over the finish surface with the blunt tool.
 9. The method as defined in claim 7 wherein deforming further comprises advancing the blunt tool into the rotating finish surface of the workpiece by about 0.03 mm beyond first contact between the rotating workpiece and the blunt tool.
 10. The method as defined in claim 1 wherein the blunt tool includes a blunt pellet operatively associated therewith, the pellet to rubbingly contact the finish surface.
 11. The method as defined in claim 10 wherein the pellet is formed from a material chosen from iron-tungsten alloys, silicon carbide, boron nitride, titanium nitride, diamond, and hardened tool steel.
 12. The method as defined in claim 10 wherein the pellet has a shape chosen from a sphere shape, a spherical cap shape, a roller shape, and a parabolic shape.
 13. The method as defined in claim 1 wherein a thickness of the nanocrystallized surface layer ranges from about 3 μm to about 15 μm.
 14. A rotational member formed by the method of claim 1 wherein the rotational member comprises a brake rotor, a brake drum, or a combination thereof.
 15. The rotational member as defined in claim 14 wherein the rendered surface is a friction surface, and wherein the friction surface exhibits hardness of between about 56 HRC and about 64 HRC. 